[Gentoo] Fixing parallel make

I’ve been trying to fix the Makefile for Gabedit, a computational chemistry GUI, for a few days. It was completely broken for parallel builds (make -j#). Finally I came across an excellent post by GNU make maintainer Paul D. Smith that allowed me to fix the problems. The key was a make-defined variable called MAKECMDGOALS, which is defined to whatever targets were set on the command line.